The present invention relates to a method of measuring the degree of degradation of a coating film on a painted material and an apparatus for carrying out the same. More particularly, the invention is concerned with a method of measuring quantitatively the corrosion preventing capability of a coating film of a painted steel material and an apparatus for carrying out the method.
Metal, material such as steel, is usually in use with a paint film coated thereon for protecting the metal material against corrosion. The coating film is degraded or deteriorated in the course of time lapse under the influence of ultraviolet rays, moisture, corrosion of substrate metal and other factors. For determining the time for recoating (repainting) as well as for the maintenance of large scale steel structures, such as bridges, buildings, factory equipment and the like and transportation vehicles or tools, such as automobiles, ships, containers and others, it is extremely important to quantitatively measure the corrosion preventing function or capability of the coating films (paint films) to thereby determine accurately and objectively the degree of deterioration thereof.
As the methods of measuring nondestructively the degree of deterioration of the films coated on the metal material, there have heretofore been know a method of measuring change in brilliance and color, a method of measuring an insulation resistance and a method of measuring AC impedance.
In conjunction with the AC impedance measuring method, it is known to measure parallel resistance and parallel capacity which balance with a system to be measured, by using AC currents of frequencies varying over a wide range, to thereby establish the relationships between the measured values and the frequencies, or determine the phase differences tan .delta. on the basis of the parallel resistance and the parallel capacity to thereby establish the relationships between the phase difference tan .delta. and the frequencies. These relations are then utilized for determining the corrosion preventing effect (as known, for example, from a Japanese periodical "Denki Kagaku or Electrochemistry in English", No. 23, 1955, pp. 15 to 18, and a Japanese periodical "Kogyo Kagaku Zasshi or Industrial Chemistry Magazine", Vol. 61, No. 3, 1958, pp. 291-295) or to measure the impedances and the phase differences over a wide frequency range covering a high frequency (10 KHz to 100 KHz) to a low frequency (1 mHz to 100 mHz) [as known, for example, from "Collection of Lectures on the 30-th Anti-Corrosion Symposium", B-212 (1983)].
These methods are adopted mainly at the level of laboratories for analyzing a sample piece for test or monitoring a test piece mounted on an actual structure. However, these methods can not be employed for evaluating the corrosion preventing capability of a coating film at arbitrary points of the actual structure because of restriction imposed on the apparatus for carrying out the method.
On the other hand, there is known a method of evaluating the corrosion preventing capability of a coating film at arbitrarily selected locations of an actual structure in accordance with the AC impedance measuring principle. According to this method, electrodes are disposed at a substrate metal material and a surface of a coating film for constituting a balanced detection circuit by inserting a filter sheet impregnated with liquid electrolyte between the electrodes to thereby measure the impedance (as known, for example, from Japanese Patent Laid-Open Application No. JP-A-54-77191, published June 20, 1979).
This method is based on the prerequisite that the coating film is uniform over the area where measurement is performed in order to obtain the characteristic value of that area through a single-point measurement. However, the coating film actually applied is inherently nonuniform in respect to the film thickness and other factors, and the degradation of the coating film progresses in such a manner as to promote the nonuniformity more seriously. This fact is not taken into consideration by the method mentioned above.
When a coating film undergone degradation to a certain extent is carefully observed, it is commonly found that the coating film is locally delaminated, rust is produced in a spot-like pattern, a small bulge is formed and/or that the film is partially chalked. Thus, it is naturally difficult to determine accurately and quantitatively the state of a coating film undergone nonuniform degradation as mentioned above through the conventional single-point AC impedance measurement.
Thus, there exists a demand for a method and an apparatus which is capable of accurately and quantitatively determining the degraded state of a coating film which tends to undergo nonuniform degradation and diagnosing quantitatively the degree or extent of degradation of a coating film applied onto a metal material, for ensuring effective operation and utilization, i.e., maintenance, updating measures, and lengthened use life of public and private facilities such as large scale steel buildings, transportation tools and the like.